Your question is very unclear. What do you mean by 'pass'? Maximum loop iteration count? That would be implementation depentent. GLSL does not define such limit.
If what you mean is rendering passes, then it's like asking how many frames of animation can GPU display.
Or perhaps you are referring to what ATI's first shaders defined - one pass is 3x texture access. There is no such limit either. The only limit is MAX_TEXTURE_IMAGE_UNITS which defines how many texture units you can use in a shader. You can attach only one texture to each unit, but you can sample from one unit many times. My "heaviest" shader sampled from one texture unit 9600 times per pixel.